Conveyor drive and indexing mechanism for liquid scintillation spectrometers



1967 R. E. CANNON CONVEYOR DRIVE AND INDEXING MECHANISM FOR LIQUIDSCINTILLATION SPECTROMETERS Filed Aprll 11, 1966 N mo T N NN A Vfi W E DN O W A R ATTORNEY United States Patent CONVEYOR DRIVE AND INDEXINGMECH- ANISM FOR LIQUID SCINTILLATION SPEC- TROMETERS Raymond E. Cannon,San Diego, Calif., assignor to Beckman Instruments, Inc., a corporationof California Filed Apr. 11, 1966, Ser. No. 541,595 9 Claims. (Cl.74-436) This invention relates to a conveyor drive and indexingmechanism of the type used for liquid scintillation spectrometers and,more particularly, to such a mechanism employing a modified Geneva drivefor providing a smooth output motion around the indexing point with ahigh degree of positioning accuracy while using a relatively low torquegear motor drive and simple mechanism.

One of two major tasks of automatic sample conveying devices used withliquid scintillation spectrometers is accurately positioning the samplesin an index position over an opening in the base of the conveyor tablefor further handling. As an example, a conveyor chain, which may bedriven to and from the index position, has used a lever arm which comesin contact with the sample holder and then actuates a limit switchshutting off the current to the associated drive motor. Examples of thistype of device are disclosed in US. Patents 3,188,468 Packard, and3,206,006-Meedcr et al. In the latter, the indexing switch is actuatedby the driving cam which serves to position the conveyor over the indexposition. This is a simple and inexpensive approach but has manydisadvantages, among which are the requirement that the drive motor beeffectively braked, mechanically with a solenoid brake, or dynamically.

Mechanical brakes, available on electric motors, do not stop with a highdegree of accuracy and solenoid brakes simply do not work reliably.Dynamic braking may be effective in stopping the motor but does not holdthe motor once it is stopped. Even if the braking problem is mastered inthis type of direct drive system, the starting and stopping of theconveyor is very abrupt since the resulting motion is linear in nature.

Indexing the chain from station to station has to be done withrepeatable accuracy and the use of limit switches actuated by mechanicalarms or toggles acting on the conveyor chain will also not performreliably, since the distance moved bythe conveyor chain is large withrespect to the sensing arm acting on it, and therefore even very smallinconsistencies in the switch or actuator reflect large errors inconveyor chain position. The precise point at which a limit switchactuates will change with use which is inherent in mostspring-controlled devices.

Another problem is that the limit switches display an adversecharacteristic termed bounce which means that when the switch isactuated the contacts make and break several times, yielding severalsignals to the associated electronics which are looking for only onesignal. This confuses the electronics to the point that additionalcircuitry must be designed to blank out all but the first signal. Limitswitch actuator arm toggles are in general also easily damaged andrequire frequent adjustment, which is undesirable as it may involve afield service call.

It is the main purpose of the invention to provide a new and improvedconveyor drive and indexing mechanism for liquid scintillationspectrometers which will give accurate indexing of the chain fromstation to station, using a low-cost modified Geneva drive capable ofdriving the conveyor in a non-linear manner and with resulting smoothmotion and accurate positioning around the index point.

This and other objects are achieved by providing a ice conveyor driveand indexing mechanism of the type used to position sample bottles in aliquid scintillation counting system which is characterized by: a drivemotor having a motor output shaft; a drive wheel having at least threeengaging mechanisms attached thereto, spaced evenly around itscircumference, said drive wheel. connected to said motor output shaftfor driving said drive wheel; a driven wheel containing at least threeslots extending inwardly from its circumference and spaced evenlytherearound for cooperating with said engaging mechanisms, said engagingmechanisms and slots being located to effectively provide a dwell timeover several degrees of rotation of said drive wheel during which thereis an insignificant amount of motion of said driven wheel; and, meansconnecting the output of said driven wheel to a conveyor drive sprocketwhereby the output motion at said driven wheel and conveyor drivesprocket is nonlinear, effectively having dwell times, and has slowacceleration and deceleration curves.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention, and further objects and advantages thereof, can best beunderstood by reference to the following description and accompanyingdrawing which illustrates an exploded isometric view of the mechanism ofthe invention, having a portion of the driven wheel cut away for clarityof illustration.

Turning now to the drawing, there is illustrated a drive mechanism,which is a four-position intermittent motion device with effectivelyshort dwell cycles and slow acceleration and deceleration curves. Itwill be understood, of course, that the invention is not limited to adevice having four positions but that any number of positions of threeor over might be employed, and it is not intended to limit the inventionthereto. However, four positions will provide a sinusoidal output motionfor a linear rotational input motion whereas other numbers of positionsdo not necessarily do so.

The drive is a modified Geneva drive, which differs from other Genevadrives in that it has no locking cam to hold it in a dwell position. Itis positively locked only at a single point, although it has a highoutput to input mechanical advantage which makes it difficult, if notimpossible, to drive the device from output to input.

The drive consists of a drive wheel 10, mounted on a motor output shaft11, through holes 12 and 13 in lower and upper mounting plates 14 and15, which contains bushings 16 and 17, respectively, for cooperatingwith the shaft 11. The shaft 11 goes through the drive wheel 10 at itscenter through a hole 18. Around the circumference of drive wheel 10there are four holes 19, posi-- tioned apart, in which an engagingmechanism in the form of pins 20 are mounted extending downwardly fromthe lower surface of the wheel 10. The wheel 10 also contains fourphotocell holes 21, positioned 90 apart and in between the holes 20.These holes 21 serve to cooperate with a lamp 22, mounted in a socket 23and by a bracket 24 and supporting column 25 to the upper mounting plate15.

Lower mounting plate 14 contains a cooperating hole 26, under the lowerside of which a photocell detector 27 is mounted. The lower and uppermounting plates 14 and 15 are connected together by means of spacers 28and, in turn, are connected to a conveyor base-plate (not illustrated)by means of spacers 29. Such a conveyor base plate is illustrated in acopending application entitled, Control Tower and Method for ProgrammingAutomatic Radioactivity Measuring SystemR. Nather, Ser. No. 541,674,filed Apr. 11, 1966, and assigned to the assignee of the presentinvention. This copending application also discloses a conveyor chain,such as may be driven by the present invention, and associated drivesprocket, guide rails and idler wheels. A typical chain is illustratedin more detail in a copending application entitled, Sample ConveyingChain for Liquid Scintillation Spectrometer- R. Cannon, Ser. No.541,843, filed Apr. 11, 1966, and assigned to the assignee of thepresent invention.

Returning to the drawing, a driven wheel 30, containing four slots 31extending inwardly from its circumference partway along radii thereof,is mounted on a shaft 32, through a central hole 33 therein. The shaft32 rides in bushings 34 and 35, which are mounted in holes 36 and 37 inlower and upper mounting plates 14 and 15, respectively. 7

A geared wheel 38 is also mounted on shaft 32 between wheel 30 andbushing 35 and contains a gear 39 near its top for rotation around theshaft 32. Gear 39 is herein illustrated as having twelve teeth. Thewheels 10, 30 and 38 are locked to the shafts 11 and 32 by the lookingpins 40, which prevent them from rotating relative to their shafts.

The shaft 11 is driven by a gear motor 41 which is made up of aundirectional drive motor 42 and associated gear box 43 connected todrive the motor output shaft 11. The motor can be bidirectional toreverse chain direction. In the illustrated embodiment, the gear motoris a 25 r.p.m. gear motor delivering a 20-inch pound torque which issufficient to drive a conveyor of the type illustrated in the abovereferenced copending applications. A driving chain 44 engages the teethof the gear 39 and goes to a second cooperating gear (not illustrated)which is connected to a conveyor driving sprocket such as illustrated inthe above referenced copending application Ser. No. 541,674. Thecooperating gear has 30 teeth. Since the pins 20' and slots 31aredesigned to provide a fourposition device upon the rotation of thewheels and 30 through one revolution, the 2 /2 to 1 gear reductionbetween the gear 39 and its cooperating gear further reduces this to 10positions during one revolution, as illustrated on the conveyor drivesprocket and indexing wheel of the aforementioned application, Ser. No.541,674.

The fact that at the index position the motion is minimal may beillustrated by considering that when index is indicated by the lamp 22shining through the holes 21 and 26 on the photocell 27, two of the pins20 are next to the-driven wheel 30, one just entering one slot 31 andthe other just leaving the next slot 31. At this time, there ispractically no motion of the driven wheel 30, and not until the pin 20entering the slot 31 proceeds substantially down the slot is thereapreciable movement of the driven wheel 30. With radial slots the motionof the driven wheel 30 is sinusoidal in nature.

The drive wheel 10 may be made of cast aluminum and the driven wheel 30may be an injection molded wheel. The device, as illustrated anddescribed, is somewhat simpler than the conventional Geneva drive,having 1 much shorter effective dwell time with no provision forpositive locking during the entire dwell time. However, it has thedegree of precision necessary to accurately perform the sample conveyingand indexing function required by the application described.

The necessity for knowing when a sample is in position :loes notdisappear with the use of the modified Geneva irive and some means ofdetermining the fact is essen- ;ial. The drive does, however, relievethe sensing device of B decision making and command responsibilitiesasking )nly that it substantiate a fact. This is possible because of theeffective dwell time inherent in the drive. When he driven wheel 30, oroutput of the modified Geneva irive, is on index, the input or drivewheel 10 can be rotated approximately 30 before any noticeable movenentoccurs at the output of driven wheel 30 in the four position embodimentdescribed above. The result is that position sensing is far lesscritical than with a linear irive.

The lamp 22 and photocell 27 provides a highly reliable means to providean electronic signal which will detect the positioning with a highdegree of accuracy due to the small amount of motion around the indexpoint.' The holes 21, in the drive wheel 10, allow the passage of lightonly when the driven wheel 30 is on index. When the photocell 27 isexposed to light from the lamp 22, its resistivity changes and thechange is detected by associated electronics (not illustrated) providingthe necessary signal.

The 2 /2 to 1 reduction between the gear 39 and its associated gear, inaddition to changing the four-position drive to ten positions at thefinal sprocket, further reduces by a factor of 2 /2 any eror in themodified Geneva drive which may be allowed by taking advantage ofcomparatively loose manufacturing tolerances which will then not affectthe resulting indexing accuracy.

The foregoing mechanism has the advantage of very gentle accelerationand deceleration and the components are subjected to far less stress andimpact than with a linear driving system. This is particularly importantwhen moving samples per minute which is a common requirement of suchsystems. Neither accurately braked motors, nor motors with holdingbrakes, need be used. The device can only be effectively driven from theinput side and force applied to the output side will not affect theindex position due to the high mechanical advantage. At the point whereone of the pins 20 is just entering a slot 31 and another pin 20 is justleaving a slot 31, and only then, is this mechanism positively locked.Normal Geneva drives are locked for about of input rotation. The presentinvention is also only effectively locked for 10 or 15 by the mechanicaladvantage, which is more than sufiicient in the present application.

Adjustment of the index points is never needed and, in fact, is notpossible with the mechanism. The indexing is established at themanufacturing level by the nature of the drive and the fixed ratio ofthe final drive with respect to the driven wheel 30. The photocellsensing device is also exceptionally reliable since there are no movingparts to wear out. One possible exception might be the lamp 22; however,it may be operated at a reduced voltage thereby extending its lifeenormously. For repetitive indexing applications, Geneva type drives areunchallenged in their superiority over other methods. For the presentapplication, the modified Geneva described herein, is even moreadvantageous than a conventional Geneva, which would have to turn fasterand would involve more lost motion, resulting in more jerky driving ofthe conveyor chain than with the present device.

Since the principles of the invention have now been made clear,modifications which are particularly adapted for specific situationswithout departing from those principles will be apparent to thoseskilled in the art. The appended claims are intended to cover suchmodifications as well as the subject matter described and to only belimited by the true spirit of the invention.

What is claimed is:

1. A conveyor drive and indexing mechanism for liquid scintillationspectrometers including:

a drive motor having a motor output shaft;

a drive wheel having at least three engaging mechanisms attachedthereto, spaced evenly around its circumference, said drive wheelconnected to said motor output shaft for driving said drive wheel;

a driven wheel containing at least three slots extending inwardly fromits circumference and spaced evenly therearound for cooperating withsaid engaging mechanisms, said engaging mechanisms and said slots beinglocated to effectively provide a dwell time over several degrees ofrotation of said drive wheel during which there is an insignificantamount of motion of said driven wheel; and,

means connecting the output of said driven wheel to a conveyor drivesprocket whereby the output motion of said driven wheel and saidconveyor drive sprocket is non-linear, effectively having dwell times,and has slow acceleration and deceleration curves.

2. The mechanism of claim 1 in which said drive wheel contains at leastthree holes evenly spaced around its circumference and in between andsubstantially equidistant from said engaging mechanisms, said holescooperating with a photocell detector and light positioned to provide anindex signal.

3. The mechanism of claim 1 in which said drive motor is a gear motor.

4. The mechanism of claim 1 in which said means connecting the output ofsaid driven wheel to a conveyor drive sprocket comprises, a gear mountedon a shaft through said driven wheel, a second gear mounted on a shaftthrough said sprocket, said gears having a tooth ratio sufficient toconvert from the number of positions indicated by the number of saidengaging mechanisms and associated slots to the number of indexpositions on said sprocket, and a chain interconnecting said gears.

5. The mechanism of claim 1 having four engaging 20 mechanisms and fourslots.

6. The mechanism of claim 5 in which said slots are along radii of saiddriven wheel.

7. The mechanism of claim 6 in which said engaging mechanisms are pinsmounted perpendicular to said drive wheel to engage said slots.

8. The mechanism of claim 7 in which said pins and slots are positionedto provide a sinusoidal output motion of said driven wheel for a linearrotational input motion of said drive wheel, said driven wheel having aneffective dwell time centered around an index position.

9. The mechanism of claim 8 in which said drive wheel contains fourholes evenly spaced around its circumference in between andsubstantially equidistant from said pins, said holes cooperating with aphotocell detector and light positioned to provide an index signal whensaid driven wheel is in said index position.

References Cited UNITED STATES PATENTS 442,404 12/ 1890 Steiger 744362,116,127 5/1938 Steiert 7484 X 2,888,833 6/1959 Toderick 7441S X2,890,590 6/1-959 OHarah 74436 X 3,024,402 3/1962 Adel 74-436 X3,106,109 10/1963 Dexter 74436 X 3,206,006 9/ 1965 Meeder et a1 198-1313,216,777 11/1965 Allen 74436 X 3,279,281 10/ 1966 Anderson et al74-'820 3,286,536 11/196-6 Hallmann 7484 DONLEY J. STOCKING, PrimaryExaminer.

L. H. GERIN, Assistant Examiner.

1. A CONVEYOR DRIVE AND INDEXING MECHANISM FOR LIQUID SCINTILLATIONSPECTROMETERS INCLUDING: A DRIVE MOTOR HAVING A MOTOR OUTPUT SHAFT; ADRIVE WHEEL HAVING AT LEAST THREE ENGAGING MECHANISMS ATTACHED THERETO,SPACED EVENLY AROUND ITS CIRCUMFERENCE, SAID DRIVE WHEEL CONNECTED TOSAID MOTOR OUTPUT SHAFT FOR DRIVING SAID DRIVE WHEEL; A DRIVEN WHEELCONTAINING AT LEAST THREE SLOTS EXTENDING INWARDLY FROM ITSCIRCUMFERENCE AND SPACED EVENLY THEREAROUND FOR COOPERATING WITH SAIDENGAGING MECHANISMS, SAID ENGAGING MECHANISMS AND SAID SLOTS BEINGLOCATED TO EFFECTIVELY PROVIDE A DWELL TIME OVER SEVERAL DEGREES OFROTATION OF SAID DRIVE WHEEL DURING WHICH THERE IS AN INSIGNIFICANTAMOUNT OF MOTION OF SAID DRIVEN WHEEL; AND, MEANS CONNECTING THE OUTPUTOF SAID DRIVEN WHEEL TO A CONVEYOR DRIVE SPROCKET WHEREBY THE OUTPUTMOTION OF SAID DRIVEN WHEEL AND SAID CONVEYOR DRIVE SPROCKET ISNON-LINEAR, EFFECTIVELY HAVING DWELL TIMES, AND HAS SLOW ACCELERATIONAND DECELERATION CURVES.